Wet strength is a desirable attribute of many disposable paper products that come into contact with water in use, such as napkins, paper towels, household tissues, disposable hospital wear, etc. In particular, it is often desirable that such paper products have sufficient wet strength to enable their use in the moistened or wet condition. Thus, the product should resist tearing, ripping, disintegration and the like such that it substantially maintains its integrity during the intended use. For example, moistened tissue or towel may be used for body or other cleaning. Unfortunately, an untreated cellulose fiber assemblage will typically lose 95% to 97% of its strength when saturated with water such that it cannot usually be used in the moistened or wet condition.
Paper products develop dry strength in part due to interfiber hydrogen bonding. When the paper product is wetted, water disrupts the hydrogen bonds and, as a consequence, lowers the strength of the paper product. Historically, wet strength of paper products has been increased primarily by two approaches. One approach is to prevent water from reaching and disrupting the hydrogen bonds, for example, by coating the paper product. Another approach is to incorporate additives in the paper product which contribute toward the formation of interfiber bonds which are not broken or, for temporary wet strength, which resist being broken, by water. The second approach is commonly the technique of choice, especially for tissue products. In this latter approach, a water soluble wet strength resin may be added to the pulp, generally before the paper product is formed (wet-end addition). The resin generally contains cationic functionalities so that it can be easily retained by the cellulose fibers, which are naturally anionic.
A number of resins have been used or disclosed as being particularly useful for providing wet strength to paper products. Certain of these wet strength additives have resulted in paper products with permanent wet strength, i.e., paper which when placed in an aqueous medium retains a substantial portion of its initial wet strength over time. Exemplary resins of this type include urea-formaldehyde resins, melamine-formaldehyde resins and polyamide-epichlorohydrin resins. Such resins have limited wet strength decay.
Permanent wet strength in paper products is often an unnecessary and undesirable property. Paper products such as toilet tissues, etc., are generally disposed of after brief periods of use into septic systems and the like. Clogging of these systems can result if the paper product permanently retains its hydrolysis-resistant strength properties. Therefore, manufacturers have more recently added temporary wet strength additives to paper products for which wet strength is sufficient for the intended use, but which then decays upon soaking in water. Decay of the wet strength facilitates flow of the paper product through septic systems. Numerous approaches for providing paper products claimed as having good initial wet strength which decays significantly over time have been suggested.
For example, U.S. Pat. No. 3,096,228, Day et at., issued Jul. 2, 1983, U.S. Pat. No. 3,556,932, Coscia et al., issued Jan. 19, 1971; U.S. Pat. No. 3,740,391, Williams et al., issued Jun. 19, 1973; U.S. Pat. No. 4,605,702, Guerro et al., issued Aug. 12, 1986, and U.S. Pat. No. 4,675,394, Solarek et al., issued Jun. 23, 1987, suggest various approaches for achieving temporary wet strength with polymers or other compounds.
While the art has provided a variety of paper products having temporary wet strength, none has provided paper products in the manner of the present invention. It is an object of this invention to provide paper products, including paper tissue products such as toilet tissue, that have an initial wet strength sufficient for use of the paper product in the moistened condition, but which also exhibit wet strength decay (i.e., temporary wet strength) such that very low strength levels are attained subsequent to the period of intended use. Another object of the present invention is to provide paper products having a combination of an initial wet strength sufficient for use of the paper product for body cleaning in the moistened condition, and a rate of wet strength decay sufficient for a flushable product. It is a further object of the present invention to provide tissue paper products having an initial total wet tensile strength of at least about 80 g/inch, preferably at least about 120 g/inch. Yet another object of this invention is to provide tissue paper products having, in addition to these initial total wet strengths, a 30 minute total wet tensile strength of not more than about 40 g/inch, preferably not more than about 20 g/inch. Another object of the invention is to provide tissue paper products having such initial wet strength and which also exhibit a wet strength decay rate after 30 minutes of soaking in neutral pit water of at least about 70%, preferably at least about 80%.